The present invention relates to a focusing device for an electron tube having permanent magnets.
In the electron tube field, such a device serves to oppose the natural divergence of the electronic beam, which could have the consequence that at least part of the latter would be intercepted by the parts of the tube traversed by it along its path. This would in turn be disadvantageous for the operation of the tube and in particular its efficiency.
This is more particularly the case with ultra-high frequency travelling wave tubes where the beam follows a long linear path between the cathode from which it is emitted and the collector by which it is collected.
The invention also relates to the electron tube equipped with such a device and in particular a travelling wave tube.
The present focusing devices are constituted by alternating permanent magnets and alternating pole pieces, fixed against one another about the vacuum envelope of the tube.
These pieces comprise flat circular washers having a hole in the centre thereof and when all of them are installed, a hollow cylinder is formed, whose axis coincides with that of the tube along which the electron beam is propagated. The magnetized washers have a magnetization perpendicular to their faces. In this way, the necessary magnetic field is obtained and its lines of force in the vicinity of the said axis have a component in the direction of the latter.
According to the prior art, the mounting of such focusing devices on the tube firstly involved the fitting of the pole pieces on the tube envelope, prior to the fitting of the electron gun, constituted by the cathode and the associated parts, of the collector and of the coupling antennas of the tube. This was followed by the magnets, previously split into two halves in order to permit their insertion between the pole pieces. The two halves in question were then fixed together by a clip to prevent them moving apart under the action of magnetic repulsion.
This procedure had a number of disadvantages. These were more particularly caused by the necessary breaking operation with the resulting demagnetizing action, the inevitable waste and the resulting loss of material. In addition, certain waste could even be attracted towards the assembly in the air gap and impair the homogeneity of the field. Another disadvantage was the necessity of a locking member for the two parts of each magnetized washer.